Generally, a color cathode-ray tube apparatus has an envelope comprised of a panel and a funnel joined to the panel to form one component. Three electron beams emitted from an electron gun arranged inside a neck of the funnel are deflected by horizontal and vertical deflection magnetic fields generated by a deflector mounted outside the funnel. While scanning horizontally and vertically, the three electron beams strike a phosphor screen formed on the inner face of the panel so as to oppose a shadow mask. Thus, the color cathode-ray tube apparatus displays color images.
In such a color cathode-ray tube apparatus, in order to display images with high resolution on the phosphor screen, it is necessary to make a spot diameter on the phosphor screen as small as possible by reducing the effect of spherical aberration through enlarging the effective lens aperture of a main lens in the electron gun.
A main lens of an electron gun used for a conventional color cathode-ray tube is described, for example, in Unexamined Japanese Patent Application No. Tokkai Hei 3-152834 and Unexamined Japanese Patent Application No. Tokkai Hei 4-133247. As shown in FIG. 14, the main lens is comprised of a focusing electrode 13 with an end face (a bottom 13c) and a final accelerating electrode 14 with an end face (a bottom 14c) adjacent to the focusing electrode 13. The end face of the focusing electrode 13 and that of the final accelerating electrode 14 oppose each other. Each end face has an oblong electron-beam through-hole having its major axis in the horizontal direction. The main lens contains field forming electrode plates 13a and 14a, each of which has three electron-beam through-holes at the position recessed from its end face.
The focusing electrode 13 is welded on a surface 74 contacting with an auxiliary focusing electrode 12 on a cathode side. The focusing electrode 13 is fixed to an insulating support 21 (made of weld glass bead) through bracket portions 75 formed on the upper and lower faces of the auxiliary focusing electrode 12 in the vertical direction or through buried portions 76 formed on the upper and lower faces of the focusing electrode 13 in the vertical direction. The final accelerating electrode 14 is fixed to the insulating support 21 through bracket portions 77 or buried portions 78 formed on its upper and lower faces in the vertical direction.
The focusing electrode 13 and the final accelerating electrode 14 are fixed to the insulating support 21 by heating the insulating support 21 to a high temperature to soften it and forcing the bracket portions 75 and 77 or the buried portions 76 and 78 into the insulating support 21.
At that time, an opening 13b of the focusing electrode 13 is subject to a force. Therefore, while the length of the opening 13b decreases in the vertical direction, the length of an opening at the bottom 13c of the focusing electrode 13 increases in the vertical direction. In this case, the field forming electrode plate 13a functions as a supporting point. Consequently, the focusing power of the main lens in the vertical direction decreases while that in the horizontal direction increases. Thus, an electron beam spot that should be focussed on a phosphor screen optimally (in a just-focusing condition) is in an over-focusing condition in the horizontal direction and in an under-focusing condition in the vertical direction. As a result, there has been a problem that the spot of an electron beam on the screen increases in diameter or is distorted.
The difference between the focus condition in the horizontal direction and that in the vertical direction varies depending on how a force is applied in forcing the focusing electrode 13 into the insulating support 21, thus causing variations in every cathode-ray tube.
The same problem may also occur in fixing the final accelerating electrode 14 to the insulating support 21.